Inductors are a fundamental electromagnetic component used in to a wide variety of devices, such as actuators, relays, motors, DC-to-DC converters and radio frequency (RF) circuits. Inductors having large inductances typically include wires wrapped around a bulk dielectric or ferrimagnetic core, and are used in power converters and relays. Radio frequency inductors having small inductances typically are helical coils having an air or ferrite core, and are used in RF circuits and communications equipment.
Inductors for the microwave region can become too small to fabricate and suffer low efficiency and Q values. Conventional RF inductor techniques are often abandoned as a result. For instance, the ferrite core, or tunable coil slug, is unusable above VHF due to eddy current losses in the ferrite. Even printed spiral inductors have limited usefulness at microwave frequencies, as magnetic field circulation through silicon substrates results in eddy-current loss, and a higher than normal parasitic capacitance.
Radio frequency (RF) magnetic materials must be nonconductive or nearly so, for the magnetic fields to penetrate. For instance, inductance drops if a solid core of pure iron or steel is placed inside a RF inductor. Yet, if the same material is finely divided into insulated particles then the inductance increases. This is the basis of pentacarbonyl iron or “powdered iron” inductor cores, in which the powder grains may have insulative coatings, and grains size not much larger than the conductor RF skin depth. Nonconductive, highly magnetic atoms are unknown at room temperature and atmospheric pressure.
RF magnetic materials may occur naturally only as lodestone or magnetite. Magnetic permeability is a phenomenon that happens inside atoms, by atomic spin while dielectric permittivity happens between atoms as the dipole moment of polar molecules. With about 100 types of atoms, the options for new magnetic materials are more limited than for dielectrics, as new types of molecules may be created more readily than new types of atoms. Magnetic effects occur inside atoms as spin physics while dielectric effects occur between atoms as dipole moment. Ferrimagnetic materials are ferrites and garnets, materials having high bulk resistivities (107 Ωm) and are usable at RF and microwave frequencies. Ferromagnetic materials are generally metallic, conductive, and unsuitable for RF applications.
The first synthetic RF ferrites have been attributed to J. L. Snoek of the Phillips Research Laboratories in the Netherlands. Magnetic materials were strategic in World War II, with German developments including isoimpedance magnetodielectrics ((μ=∈)>>1)) (“Schornsteinteger” (Chimney Sweep), H. A. Schade, U.S. Naval Technical Mission To Europe, Tech Rep. 90-45 AD-47746, May 1945).
Nickel zinc ferrite cores typically offer high efficiency for a relatively small inductor. However, nickel zinc ferrite is not a perfect insulator. Eddy currents may form due to partial conductivity and resistance losses are exhibited as heat.
U.S. Pat. No. 5,450,052 to Goldberg, et al. is entitled “Magnetically variable inductor for high power audio and radio frequency applications”. The patent discloses a magnetically variable inductor for high power, high frequency applications which includes a solenoid with a magnetic core therein, disposed coaxially around a conductor for carrying the high power, high frequency signal, and a variable current source coupled with the solenoid so that a manipulation of the current through the solenoid results in a variable inductance for the conductor.
There exists a need for an inductor with lower losses, higher Q and efficiency. With radio communications moving to higher and higher frequencies, the need is becoming ever more acute. A typical RF communication device, such as a cellular telephone may use more than 20 inductors.